After a semiconductor wafer has gone through processing to form connections for semiconductor devices, the semiconductor devices are generally encapsulated with a molding material, for example a resin, in order to protect the semiconductor devices from physical and environmental damage. In order to form the resin into a desired shape and size, semiconductor substrates are placed into one or more cavities and the resin is introduced into the one or more cavities such that the resin is molded into the desired shape and size over the semiconductor substrates.
Compression molding and transfer molding are molding techniques commonly used to coat semiconductor substrates with resin. It is possible both in the case of compression molding and in the case of transfer molding, for a molding system to comprise a plurality of molding presses, each molding press comprising one or more cavities.
In the case of compression molding, a molding material, which may take the form of preheated paste or pellets, is introduced into the open, usually heated cavity of the molding press, which is subsequently closed. The molding material is pressed by the molding press under pressure, and the molding material melts and fills the cavity completely. The molding material remains in the cavity until it has cured.
In the case of transfer molding, a defined quantity of the molding material, typically a thermosetting material, is liquefied by heat and pressure, and then forced into the one or more cavities through an inlet, and is held there under heat and pressure until the resin has solidified. The molding material is usually introduced into each cavity simultaneously from a supply container through branched feeding lines.
The walls of the one or more cavities are typically heated to a temperature which is above the melting temperature of the molding material in order to obtain appropriate viscosity of the molding material within the one or more cavities. Despite that, pockets of air or gas may form in the molding material in the one or more cavities, especially in regions in the one or more cavities that are particularly intricate or narrow thereby creating voids in the molding material. Voids may cause the semiconductor device to malfunction, and the yield rate of the semiconductor devices to drop.
One technique of reducing or preventing the formation of voids is to use vacuum to assist the molding process. FIG. 1 shows a conventional multi-press molding apparatus or system 10 comprising a plurality of molding presses 12, and a respective vacuum pump 14 connected to each molding press 12. Each vacuum pump 14 would pump out the air from and reduce the pressure in the respective molding press 12 connected to it. For example a first vacuum pump 14a would pump out the air from and reduce the pressure in a first molding press 12a, thus creating a vacuum in one or more cavities of the first molding press 12a. 
If the first vacuum pump 14a should malfunction during the molding process, there would be no vacuum (atmospheric pressure) or an inadequate vacuum (pressure substantially higher than the target molding process pressure) in the one or more cavities of the first molding press 12a. This would result in the semiconductor devices molded in the first molding press 12a being of a lower quality, for example due to the formation of voids in the molded semiconductor devices. The semiconductor devices molded in the first molding press 12a would have to be discarded, and this results in wastage and higher costs.